Molecular absorption spectroscopy is a technique that uses the interaction of energy with a molecular species to qualitatively and/or quantitatively study the species, and/or to study physical processes associated with the species. The interaction of radiation with matter can cause redirection of the radiation and/or transitions between the energy levels of the atoms or molecules. The transition from a lower level to a higher level with an accompanying transfer of energy from the radiation to the atom or molecule is called absorption. When molecules absorb light, the incoming energy excites a quantized structure to a higher energy level. The type of excitation depends on the wavelength of the light. Electrons are promoted to higher orbitals by ultraviolet or visible light, vibrations are excited by infrared light, and rotations are excited by microwaves. The infrared (IR) region is generally considered as extending from just beyond the red visible region (˜0.7 μm to 50 μm). The 0.7 to 2.5 μm region is generally called the near-infrared (NIR), the 2.5 to 15 μm region is referred to as the mid-infrared and the 15 to 50 μm region is called the far-infrared. The wavelengths of IR absorption bands are characteristic of specific types of chemical bonds, and IR spectroscopy finds its greatest utility in the identification of organic and organometallic molecules.
The data that is obtained from spectroscopy is called a spectrum. An absorption spectrum shows the absorption of light as a function of its wavelength. The spectrum of an atom or molecule depends on its energy level structure. A spectrum can be used to obtain information about atomic and molecular energy levels, molecular geometries, chemical bonds, the interactions of molecules, and related processes. Often, spectra are used to identify the components of a sample (qualitative analysis). Spectra may also be used to measure the amount of material in a sample (quantitative analysis). An instrument which measures an absorption spectrum is called a spectrometer.
Gaseous molecules are found only in discrete states of vibration and rotation, called the rovibrational state. Each such state, identified by quantum numbers describing both the vibration and rotation, has a single energy which depends on the quantum numbers. In dipole transitions described above, a single photon of radiation is absorbed, transforming the molecule from one ro-vibrational state to another. As the energies of the ro-vibrational states are discrete, so too are the energies of the transitions between them. Therefore, a photon must possess a specific energy to be absorbed by a molecule to transform the molecule between two given ro-vibrational states. Since the energy of a photon is proportional to the frequency of the radiation, (or equivalently, inversely proportional to its wavelength), only discrete frequencies (wavelengths) can be absorbed by the molecule. The set of discrete frequencies (wavelengths), often called absorption lines, at which a particular species of molecule absorbs, is called the absorption spectrum of a molecule. The width in frequency (wavelength) of each absorption line depends on the specific ro-vibrational transition, the pressure and temperature of the gas containing the molecule, and the presence (or absence) of other types of molecules in the gas. Each species of molecule has a unique absorption spectrum, by which the species of molecule may be identified. Since the energies of different rotational states of a gaseous molecule are typically spaced much more closely than the energies of different vibrational states, then the absorption lines occur in sets, each set corresponding to a single vibrational transition, and many rotational transitions. These sets of absorption lines are called absorption bands.
In the NIR, all the vibrational transitions are harmonics of fundamental, mid-infrared bands. These harmonics can be a hundred to ten thousand times weaker than their mid-infrared counterparts. Standard methods, such as Fourier Transform Infrared Spectroscopy (FTIR), commonly used to characterize mid-infrared transitions, normally have difficulty detecting these weak absorption features in the NIR spectral region. Therefore, more sensitive detection methods are required to measure NIR absorption features. Moreover, because overtone bands and combinations of overtone bands often overlap in wavelength (frequency), the NIR is normally filled with dense bands of absorption lines. It is therefore not uncommon to find spectral regions where the same molecular species has both strong and weak transitions that are co-located in wavelength (frequency). Hence, spectral resolution is very important, especially for near-infrared detection systems.
Measuring the concentration of an absorbing species in a sample is accomplished by applying the empirical Beer-Lambert Law. The Beer-Lambert law (or Beer's law) is the linear relationship between absorbance and concentration of an absorbing species. The general Beer-Lambert law is usually written as:A(λ)=α(λ)L=Cε(λ)L  (1)where A(λ) is the measured absorbance, α(λ) is a wavelength-dependent absorption coefficient, ε(λ) is a wavelength-dependent extinction coefficient, L is the path length, and C is the analyte concentration.
Experimental measurements are usually made in terms of transmittance (T), which is defined as:T=I/I0where I is the light intensity after it passes through the sample and Io is the initial light intensity. The relation between A and T is:A=−log T=−log(I/Io)  (2)
However, modern absorption instruments usually display the data as transmittance, %-transmittance, or absorbance, as a function of wavelength (or wave number). An unknown concentration of an analyte can be determined by measuring the amount of light that a sample absorbs, and then applying Beer's law. Equations (1) and (2) show that the ability of a spectrometer to detect a specific concentration depends not only on the path length through the sample, but also on the intensity noise of both the light source and the detector. Sensitivity can be quantified as a minimum detectable absorption loss (MDAL), i.e., the normalized standard deviation of the smallest detectable change in absorption. MDAL normally has units of cm−1. Sensitivity can also be defined as the achievable MDAL in a one second measurement interval, and has units of cm−1 Hz−1/2. Sensitivity accounts for the different measurement speeds achieved by diverse absorption-based methods and is a figure of merit for any absorption-based spectroscopic technique.
Typically, a spectral feature (called an “absorption peak”) of the target species is measured in order to obtain its concentration. Although most species will absorb light at more than one wavelength, the total spectral profile of any particular species is unique. The ability of a spectrometer to distinguish between two different species absorbing at similar wavelengths is called selectivity. Because spectral features narrow as the sample pressure is reduced, selectivity can be improved by reducing the operating pressure. However, the spectrometer must still be able to resolve the resulting spectral lines. Thus, selectivity ultimately depends on spectral resolution. Spectral resolution, typically measured in frequency (MHz), wavelength (picometers) or wave numbers (cm−1), is an important figure of merit for a spectrometer
Optical detection is the determination of the presence and/or concentration of one or more target species within a sample by illuminating the sample with optical radiation and measuring optical absorption by the sample and a wide variety of optical detection methods are known. Most of these methods, however, have limited resolution, and can frequently not achieve sufficient selectivity in spectral measurement. For example, FTIR spectrometers can provide a very broad spectral tuning range but only at the expense of spectral resolution. Many FTIR spectrometers cannot resolve individual rotational lines in an absorption band at low operating pressures (below ˜100 Torr). Non-dispersive infrared (NDIR) instruments have even less resolution that FTIR (typical filters cannot resolve individual rotational lines, let alone absorption bands) but enable inexpensive instruments. Tunable diode laser based absorption spectrometers (TDLAS), can achieve excellent wavelength resolution by finely tuning the laser. However, their precision and accuracy depend on being able to measure, and hence control, the laser wavelength. A TDLAS spectrometer is only as good as its wavelength measuring component. Cavity enhanced optical detection entails the use of a passive optical resonator, also referred to as a cavity, to improve the performance of an optical detector. Cavity enhanced absorption spectroscopy (CEAS) and cavity ring down spectroscopy (CRDS) are two of the most widely used cavity enhanced optical detection techniques. Cavity enhanced methods, like TDLAS depend in resolution on the quality of the wavelength monitoring device and resulting laser control. Both methods, however, provide a significant improvement in sensitivity over traditional TDLAS.
The intensity of single-mode radiation trapped within a passive optical resonator, called the ring-down cavity (RDC), decays exponentially over time, with a time constant τ, which is often referred to as the ring-down time. In practice, it is desirable to ensure that only a single resonator mode has an appreciable amplitude, since excitation of multiple resonator modes leads to multi-exponential radiation intensity decay (i.e., multiple time constants), which significantly complicates the interpretation of measurement results. The ring-down time τ depends on the cavity round trip length and on the total round-trip optical loss within the cavity, including, of course, loss due to absorption and/or scattering by one or more target species within a sample positioned inside the cavity. Thus, measurement of the ring-down time of an optical resonator containing a target species provides spectroscopic information on the target species. Both CRDS and CEAS are based on a measurement of τ.
In CRDS, an optical source is usually coupled to the resonator in a mode-matched manner, so that the radiation trapped within the resonator is substantially in a single spatial mode. The coupling between the source and the resonator is then interrupted (e.g., by blocking the source radiation, or by altering the spectral overlap between the source radiation and the excited resonator mode). A detector typically is positioned to receive a portion of the radiation leaking from the resonator, which decays in time exponentially with time constant τ. The time-dependent signal from this detector is processed to determine τ (e.g., by sampling the detector signal and applying a suitable curve-fitting method to a decaying portion of the sampled signal). Note that CRDS entails an absolute measurement of τ. Both pulsed and continuous wave laser radiation can be used in CRDS with a variety of factors influencing the choice. The articles in the book “Cavity-Ringdown Spectroscopy” by K. W. Busch and M. A. Busch, ACS Symposium Series No. 720, 1999 ISBN 0-8412-3600-3, including the therein cited references, cover most currently reported aspects of CRDS technology.
Single spatial mode excitation of the resonator is also usually employed in CEAS, but CEAS differs from CRDS in that the wavelength of the source is swept (i.e., varied over time), so that the source wavelength coincides briefly with the resonant wavelengths of a succession of resonator modes. A detector is positioned to receive radiation leaking from the resonator, and the signal from the detector is integrated for a time comparable to the time it takes the source wavelength to scan across a sample resonator mode of interest. The resulting detector signal is proportional to τ, so the variation of this signal with source wavelength provides spectral information on the sample. Unlike CRDS, the CEAS technique entails a relative measurement of τ. The published Ph.D. dissertation “Cavity Enhanced Absorption Spectroscopy”, R. Peeters, Katholieke Universiteit Nijmegen, The Netherlands, 2001, ISBN 90-9014628-8, provides further information on both CEAS and CRDS technology and applications CEAS is also discussed in a recent article entitled “Incoherent Broad-band Cavity-enhanced Absorption Spectroscopy by S. Fiedler, A. Hese and A, Ruth Chemical Physics Letters 371 (2003) 284-294.
In cavity enhanced optical detection, the measured ring-down time depends on the total round trip loss within the optical resonator. Absorption and/or scattering by target species within the cavity normally accounts for the major portion of the total round trip loss, while parasitic loss (e.g., mirror losses and reflections from intracavity interfaces) accounts for the remainder of the total round trip loss. The sensitivity of cavity enhanced optical detection improves as the parasitic loss is decreased, since the total round trip loss depends more sensitively on the target species concentration as the parasitic loss is decreased. Accordingly, both the use of mirrors with very low loss (i.e., a reflectivity greater than 99.99 percent), and the minimization of intracavity interface reflections are important for cavity enhanced optical detection. Although the present invention will be described primarily in the context of CRDS, it should be understood that the methodology is also applicable to CEAS. In other techniques, called integrated cavity output spectroscopy (ICOS) and off-axis ICOS, the cavity is aligned so as to create a set of densely spaced modes, and with these techniques the method of this invention cannot be used. The present invention relies on the RDC clearly defining a comb of equally spaced modes (having the same transverse mode number and separated by the free spectral range). Such a comb of frequencies does not exist for ICOS cavities.